This invention generally relates to methods for assembling a fiber-reinforced plastic (FRP) structure by adhesive bonding of respective FRP subcomponents. In particular, the invention relates to methods for ensuring that the adhesive bond between two FRP subcomponents has adequate strength.
Adhesive bonding is an important joining method for aerospace structures. Strong, durable bonded joints are created by proper selection of the materials (adherends and adhesive), processing, assembly and cure. The certification of the bond requires that the strength be validated. Methods are needed to validate that bond strength measurement techniques are in calibration.
Validation of the bond strength involves a combination of process control validation and final bond quality validation. The development and implementation of bond quality validation, that returns an estimate of the bond strength, requires standards (also referred to herein as “test specimen assemblies”) containing controlled levels of bond strength for calibration. A critical issue is that the weak bond standards be constructed without physical features or characteristics that can be detected by standard nondestructive inspection (NDI) methods such as ultrasound, infrared, shearography or x-ray. These standard NDI methods are performed on bonded structure to validate assembly issues and find unbonded regions. However, they are not necessarily capable of detecting weak bonds. Alternative inspection approaches for weak bond detection are needed and must be applicable to bonds that would be acceptable by the standard NDI processes. The standard needs to be constructed in a repeatable manner so that, as required, additional standards can be made. Further, the standard needs to possess variable strength bonds from weak to full strength. Finally the standard should be adaptable to the adherend thickness used in the actual construction of the adhesive joint of interest.
Such a standard would be useful for testing NDI methods of any type to determine whether the method is possibly sensitive to a weak bond interface. For the inspection methods that test for strength using loading of the bond in the testing, the standard will be mechanically failed as part of the testing and will therefore need to be replaced frequently
Weak bonds have been found in practice due to variation in the manufacturing technique. In particular, incorrect material, surface preparation and contamination are key variables that can create weak bonds that are not detectable by NDI methods. Other processes, such as incorrect assembly or curing can result in features or material change effects that can be detected by NDI techniques. The creation of useful weak bond standards therefore resides in finding a controlled manner of degrading the interface for adhesion without creating features that are detectable by standard NDI methods. Thus the weak bond standards should represent the case of bonds that pass standard ultrasonic inspection but do not have full strength. It would desirable to have a range of strengths such as one third, two thirds and full strength or 25, 50, 75 and 100% of full strength in the standards for calibration of the bond strength test method.
Weak bonds have been created in the past by adding chemical mixtures, distributing contaminates or disrupting a surface. Known methods of creating weak bonds can be difficult to repeat or will have features that can be detected by standard NDI methods. For example, poly film and aged adhesive methods can create weak interfaces, but the interface degradation is detectable by standard NDI methods. Variable grit blast methods have also been successful in creating variable strength bonds. The surface condition however is such that it is also possible with detailed inspections to detect the surface feature with standard NDI methods.
The problem to be solved was that, if a system were developed that could detect weak bonds, how could the system be calibrated and how would one know that it was operating correctly to detect a weak bond if one should exist.